A study tracks how energy and materials flow through the world's biggest cities.

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Over the last century, the explosion of the human population has been accompanied by the growth of cities worldwide. The increased presence of large cities presents unique global challenges regarding environmental sustainability.

Megacities, those containing more than 10 million people, are the largest metropolitan complexes in the world. There were only eight of them in 1970, but there are now 27. Megacities are often perceived as having high levels of poverty and pollution, but the category also contains some of the wealthiest cities in the world. They are viewed as prime candidates for the generation of innovative, sustainable solutions to reduce global environmental burdens.

Recently, a team of scientists quantified the energy and material flows that power the world’s 27 megacities using their 2010 populations. This study was focused on understanding the use of energy, water, concrete, steel, and waste, as its authors tried to quantify the economic and physical characteristics that influence resource flows at multiple scales.

For context, the 27 megacities had a combined population of 460 million in 2010, equal to 6.7 percent of the global population. But they had a gross domestic product of 14.6 percent, substantially above what you'd expect from their population alone.

The cities' water consumption was lower than the population would lead you to believe, at only 3.0 percent of global usage. Total water consumption is highest in New York (10.9 million MegaLiters), which was followed by Guangzhou (9.8 million ML), Shanghai (9.75 million ML), and Los Angeles (6.2 million ML). In other megacities, water use ranges from 0.28 million ML in Jakarta to 4.19 million ML in Tokyo.

For most types of energy, usage was consistent with scaling laws observed for cities over a wide range of populations. Megacities consume 26,347 PetaJoules, which is 6.7 percent of the global energy use—roughly in line with the percentage of the global population that lives there. Megacities consume 9.3 percent of global electricity and 9.9 percent of global gasoline. Annual energy consumption in megacities for 2011 ranged from ~78 PJ for Kolkata to ~2,824 PJ for the New York Metropolitan Area.

Megacities account for 12.6 percent of the total global waste production, a number in line with their percentage of the global GDP. The higher economic activity in larger cities results in high quantities of goods and materials that eventually leave the city as wastes. New York has the highest solid waste production in both absolute and per capita terms.

The scientists were also able to gain some understanding of factors that underlie the energy and material flows through megacities. Scientists found per capita electricity use in megacities to be significantly correlated with urbanized area per capita. The team suspects that lower-density megacities like LA and New York have greater building floor space per capital, resulting in higher electricity consumption for lighting and other building applications. (Microscale analysis demonstrates that electricity is strongly correlated with building floor area.)

The rapid growth exhibited by megacities makes accessing resources a challenge. More than half of the 27 megacities grew by more than 10 percent over the 10 year period ending in 2011. Resource flows for many of the megacities grew faster than the rates of population growth. Six megacities exhibited a doubling of electricity consumption over that decade; nine others exhibited an increase in electricity usage that was more than three times the rise in population. Analysis also revealed that growth in electricity use and transportation fuel use are significantly correlated with growth in GDP, while water use and solid waste production are not.

Clearly these 27 megacities represent a large percentage of the global resource use. While the rapid growth in population may stress city resources, there is a clear need to develop sustainable solutions in the megacities.